There has been known a fluid coupling which has a pump impeller connected to a drive shaft and a turbine impeller connected to a driven shaft and transmits power from the drive shaft to the driven shaft via fluid filled in a casing.
In case of performing a rotational speed control by means of a fluid coupling, the rotational speed on a load side can be steplessly changed from a minimum rotational speed to a maximum rotational speed by using a scoop tube, or a minimum rotational speed or a maximum rotational speed on a load side can be obtained by feeding a working fluid such as working oil to a coupling section of the fluid coupling or interrupting the feeding of the working fluid to the coupling section.
In either case, when the driven shaft is rotated at a maximum rotational speed, the maximum rotational speed of the driven shaft is normally lower than an input rotational speed of the prime mover such as a motor or an engine by about 2 to 3% due to slippage. This slippage cannot be avoid d when the rotational speed on the load side is controlled by the fluid coupling.
Specifically, when the coupling section of the fluid coupling is filled with the working fluid up to 100%, the rotational speed of the driven shaft reaches 97 to 98% of the rotational speed of the drive shaft, and hence power loss is no more than 2 to 3% due to slippage of the rotational speed. Therefore, special measures for reducing the power loss caused by slippage have not been taken.
However, energy-saving has been highlighted in recent years, and slippage between the drive shaft and the driven shaft in the fluid coupling is considered to be the subject of the energy-saving, and hence there is a demand for eliminating such slippage generated when the fluid coupling is operated at a high rotational speed, thus achieving energy-saving.